4 University of California Puhlications in Zoology [Vol. 21 



KEY TO SUBGENEEA AND SPECIES OF MICMOTUS 

 IN CALirOENIA 



1. Plantar tubercles 6 Microtus 



2. Tail very long, equaling about 34 per cent of total length; sides 

 grayish, more so than back. 



3. Incisive foramina of skull reaching greatest width at point anterior 



to middle Microtus mordax 



2'. Tail medium, equaling 32 per cent or less, of total length. 



4. Incisive foramina of skull narrowly constricted posteriorly 



Microtus montanus 



4'. Incisive foramina of skull wide open, not constricted posteriorly 



JVIicrotus calif omicus 



1'. Plantar tubercles 5. 



5. Tail very short, equaling about 20 per cent of total length; color 

 light grayish or buflfy; third lower molar with all triangles closed. 



Lagurus curtatus 



5'. Tail moderately short, equaling about 28 per cent of total length; color 

 dark brown; third lower molar normally without closed triangles. 

 CMlotus oregoni 



KEY TO SUBSPECIES OF MICBOTUS CALIFOENICUS page 



1. Size large, total length of old adults at least 190 mm., and usually over 

 200 mm.; bullae large. 

 2. Colors dark, upperparts strongly lined with blackish overhairs. 

 3. Longitudinal ridges of skull well developed. 



4. Upperparts usually very dark, in some specimens iridescent black; 



feet large, about 24 mm aestuarlnus 15 



4'. Upperparts dark grayish; feet smaller, about 23 mm vallicola 21 



3'. Longitudinal ridges of skull less conspicuously developed. 



5. Skull very large (condylobasal length about 32 mm.)....moliavensis 29 

 5'. Skull smaller (condylobasal length about 30 mm.), but zygomatic 



width proportionally greater scirpensis 24 



2'. Colors bright, reddish or brownish; skull large (condylobasal length 

 about 31 mm.). 

 6. Longitudinal ridges of skull well developed. 



7. Upperparts buckthorn brown neglectus 31 



7'. Upperparts snuff brown mariposae 19 



6'. Longitudinal ridges of skull feebly developed. 



8. Upperparts reddish huperuthrus 34 



1'. Size medium or small, total length of old adults seldom greater than 



190 mm., and usually less; longitudinal ridges normally poorly 

 developed. 

 9. Bullae large or medium. 



10. Second upper molar with posterior internal lobe; feet large, about 



25 mm.; upperftarts tawny-olive or buffy Jsemensis 26 



10'. Second upper molar without posterior internal lobe; feet smaller, 



about 22 mm.; upperparts Brussels brown calif ornicus 5 



9'. Bullae small; colors dull. 



11. Skull small (condylobasal length about 28 mm.) and narrow 



constrictus 9 



11'. Skull larger (condylobasal length about 30 mm.) and relativelv 



wider eximius 12 



